In his first 100 days in office, Mexico’s new president, Andrés Manuel López Obrador, seems intent on steering the country toward an idealistic, but ultimately ruinous, future.

Known as AMLO, the new president is fast concentrating power and influence in his own hands. He has rewarded party loyalists and mighty state bosses, enriched loyal backers like the teachers unions, stacked the courts with cronies and dried out funds for democratic institutions, weakening checks and balances.

All these steps are out of old Mexico’s playbook. Although he long ago split from the Institutional Revolutionary Party, or PRI, that’s where young AMLO initially learned his craft. The oxymoronically named party ruled Mexico for 71 years, as long as the Communists ruled Russia.

Revolutions aside, the PRI institutionalized state corruption so deeply that subsequent presidents failed to uproot it after 2000, when, for the first time since 1919, the party first lost an election. After two stints out of the presidency, a reformed PRI returned to power — with dismal results.

Finally last year, after two previous ­attempts at the top office, in came AMLO, now heading his own party and promising to undo all that was bad in Mexico. His July election victory was supposed to mark the dawn of a new era, even though his left-leaning positions hark back to bygone days of idealistic socialism, and even though his PRI-like politics are just as retro.

If ever there was a laboratory to test Margaret Thatcher’s classic observation that sooner or later socialists run out of other people’s money, AMLO’s Mexico is it.

Since he took over, foreign and local investment has dried up, and economic growth has screeched to a halt, as have tax collections. He banned all foreign investments in PEMEX, the government’s oil monopoly, and when Standard & Poor’s then downgraded PEMEX’s credit rating, he denigrated S&P’s credentials.

AMLO’s most ambitious move so far was scrapping his predecessor’s $13 billion plan to build a new airport in Mexico City. To do that, he relied on a dubious poll conducted by his Morena party’s pollsters. However popular the killing of the project may have been, the president has yet to follow up with an alternative to ease the city’s traffic congestion and modernize the international air hub.

Will AMLO’s public support — he ­enjoys up to 85% approval in national polls — last? As PEMEX and other major economic giants stumble, economists predict the state coffers, which AMLO depends on to enact his grandiose promises, will soon be empty.

Then what?

At least one dire prediction hasn’t (yet) come true: Mexico’s populist leader and America’s, President Trump, have yet to clash. On the contrary, to date Mr. Trump has had nothing but praise for AMLO, whom he often refers to as a “terrific person.”

And why not? Mexico is cooperating on Mr. Trump’s main issue, ensuring caravans from Central America stay in Mexico, rather than mob the southern border — even as resentment against the intruders among Mexicans heats up. What about the border wall? AMLO diplomatically describes it as an “internal issue” that Mexico has no say in.

AMLO has also beefed up his anti-drug bona fides by creating a new National Guard, composed of police and army units, to fight crime. Yet Mexico’s hallmarks — violence and corruption — are likely to plague the new force, even as drug-war-related violence returns.

There were 2,452 homicides in January, the highest kill rate for that month since Mexico started keeping records. After ­decades of futility — not to mention hundreds of thousands of deaths — Mexico’s U.S.-financed drug wars have yet to show many positive results.

Counting AMLO as an ally is a ­major mistake. Alone among Latin America’s Lima Group of democratic countries, Mexico under AMLO reversed its prior stance and declined to recognize Juan Guaidó as Venezuela’s provisional president, siding instead with the thug Nicolás Maduro and his Cuban backers.

AMLO’s defiance of a global front of free societies harms America’s efforts to confront the hemisphere’s Chavistas. It also signals AMLO’s comfort with America’s rivals and adversaries like Cuba, China, Russia, and the lot.

Most crucially, it shows AMLO may soon turn Mexico, our most important Latin ally, into one of them.

Twitter: @BennyAvni. From the New York Post.